Ubuntu 9.04 disabled the semi-crucial keyboard shortcut, Ctrl+Alt+Backspace, that used to force the X graphical server to restart, and made it wonky to re-enable. Luckily, Ubuntu 9.10, due out in nine days, has a single setting to restore it.
The I am a Ubuntu blog pins down a little check box, under the System->Preferences->Keyboard settings, that restores the hard-reset shortcut familiar to long-time Linux users. Head to the Layouts tab under Keyboard settings, click the "Key sequence to kill the X server" option to expand it, then check "Control + Alt + Backspace" to set it.
This might be a general change in the latest GNOME desktop software distributed across many Linux variants, so if you see a similar option in Fedora, openSUSE or elsewhere, let us know in the comments.